When a large, fully loaded semi-trailer (which weights about 80,000 lbs.) descends a 6.degree. downgrade at highway speeds for one or two miles, some form of braking other than the trucks own service brakes are generally required to avoid a runaway. Although some of the potential energy of the truck is absorbed by engine work, aerodynamic drag, rolling resistance, etc., the service brakes and/or retarder will be called upon to absorb about 400 hp at 40 m.p.h. and 700 hp at 80 m.p.h. While service brakes can apply enough torque to skid the wheels initially, their effectiveness decreases rapidly as they absorb energy and heat up. The vehicle may then regain speed requiring more braking resulting in further brake loss. An on-off, systematic application of the service brakes may be sufficient to control the semi without overheating, but there is still a fairly great risk of fade and complete brake loss.
Numerous braking systems have been developed to assist the truck's service brakes, the most notable of which is the "Jacob's brake" which is described in U.S. Pat. No. 4,572,114. The Jacob's brake is a compression release engine retarder which temporarily disables the normal operation of the engine and converts the standard four stroke cycle engine into an air compressor for developing a retarding horsepower. Although representing a significant addition to the service brakes of a vehicle, the compression release engine retarder has serious drawbacks. First, the compression release brake, and similar systems interfaced with the engine, produce a force which is limited by the size and rpm capability of the engine. These brakes can only provide the needed amount of retardation force over long and/or steep grades with the vehicle at a relatively slow speed and with the compression release brake operated at a high engine r.p.m. Once engaged, it is difficult if not impossible to shift into a lower gear.
Another system, the hydraulic retarder, has a large power capability and is easy to control. However, dissipation of the braking energy is commonly done through the vehicle's own cooling system. This severely limits the retarder performance, complicates the installation and makes these systems more expensive than the compression release system.
Yet another system, the electrical brake system, though providing a noticeable braking addition, has become unpopular due to cost, weight, fade and inertia factors, especially on the larger on-highway trucks.
Another vehicle braking system which is not interfaced with the engine is disclosed in Bultmann, U.S. Pat. No. 4,114,734. The Bultmann device has two oppositely rotating and opposing rotors coupled directly to the drive shaft of a vehicle. The Bultmann device, however, requires a separate cooling system to dissipate the heat generated by the rotors. It also is not capable of producing a constant or operator regulated retardant force.
Under current EEC legislation (European Economic Community), laden vehicles shall be tested in such a manner that the vehicle's braking system be able to withstand a braking input on a 6% downward grade for 6 km at 30 kph. What is needed is a device which meets this retarding force at all speeds down to zero, without face, in continuous operation and without service brake assistance. Furthermore, the device should be non-intrusive upon the engine, compact and inexpensive, applicable to all vehicles regardless of their engine size and be capable of integration into the vehicle transmission and/or the axle systems.